A procedure essential to all molecular biology laboratories is the detection and imaging of macromolecules. This is used in protein assays, DNA sequencing, gene mapping, and any number of other experiments and determinations. The most common method used is by tagging or labelling the molecules of interest with a radioactive species, then recording an autoradiographic image of the radioactive emission on x-ray film.
X-ray films have their limitations. The dynamic range of a typical x-ray film is about fifty-fold, which limits the degree to which one can obtain quantitative information from the film. Also, long exposures are generally required to obtain a satisfactory image, due to the limited sensitivity of the film to the .beta.-particle emissions used in most radioactive labels. In addition, variability is potentially introduced in the development of the film, since this requires a number of steps involving unstable solutions.
Recently, electronic methods of emission detection and recording, such as those employing phosphor screens (see, e.g., U.S. Pat. No. 4,684,592 to Matsuda et al., U.S. Pat. No. 4,788,434 to Takahashi et al., and U.S. Pat. No. 4,801,806 to Nakamura et al.), have enjoyed greater use in the detection and imaging of macromolecules and other labeled biological substances. These methods offer several advantages over x-ray film detection and recording methods. First, the data obtained from detecting and imaging emitted radiation may be stored on magnetic or optical media such as computer hard drives, floppy disks and CD ROMs which offer greater ease of storage as they are far less bulky and heavy than x-ray films. Second, electronically stored images may be analyzed and manipulated using computers. Several public domain and commercial software applications currently exist for the manipulation of electronically recorded images. One such public domain program is Image, available from the National Institutes of Health. Using such software, the information contained in the electronically recorded image can be analyzed at a far greater level of detail than the information available in a conventional x-ray film image.
Unfortunately, current phosphor screens suffer from serious drawbacks. One drawback is that phosphors are easily damaged by external factors such as moisture and physical abrasion. Moisture and high humidity are problems as the reaction of water with the phosphor components causes chemical deterioration of the phosphor. Physical abrasion is also a problem as samples frequently cause contamination of the screen surface which then must be cleaned physically in order to remove the contaminant.
Attempts to alleviate these problems have included the use of protective coatings on the phosphor screen. Typical of such coatings are those described in U.S. Pat. No. 4,684,592. which describes a polymer coating applied with solvent which is dried to leave a 10 .mu.m protective coating on the screen. Other coatings have been made as thin as 7.5 .mu.m. Mylar covered protective screens have also been used. Typically mylar as thin as 0.5 mils is applied to the phosphor screen with an adhesive having a thickness of approximately 1.0 mils. Unfortunately, these coatings are far too thick to allow sensitive detection of weakly emitting labels such as .sup.14 C and .sup.3 H. Current mylar screens having 0.6 to 1.0 mils of adhesive can decrease .sup.3 H sensitivity by 21,000- to 730,000-fold and .sup.14 C sensitivity by 3.3- to 5.0-fold.
Another problem is that present coating techniques do not compensate for the surface deformities on the phosphor screen, i.e., depressions and elevations, which arise from the highly particulate nature of the phosphor. Generally, coatings are deposited in greater amounts in the depressions of the phosphor screen surface compared with the elevations. The difference in coating depth may vary by as much as 10 to 20 .mu.m using conventional coating processes. Such a wide disparity in coating thickness is unacceptable for imaging radiation from very weak emitters such as .sup.3 H, for which signal attenuation by as much as 50% occurs at a coating thickness of only 1.0 .mu.m, as the regions having less coating thickness will be more sensitive to emissions than regions having greater coating thickness. Thus, the fidelity of imaging weak emitters by current phosphor screens is severely degraded with present coating techniques. Yet, low-energy radioactive labels continue to be attractive in terms of decreased exposure hazards and enabling additional research possibilities. Thus, there is a pressing need to provide a protective coating which protects the phosphor from external damage while allowing maximum sensitivity to low level radiation emitting labels.